1. Field of the Invention
This invention is directed to coatings for implantable medical devices, such as stents.
2. Description of the State of the Art
Percutaneous transluminal coronary angioplasty (PTCA) is a procedure for treating heart disease. A catheter assembly having a balloon portion is introduced percutaneously into the cardiovascular system of a patient via the brachial or femoral artery. The catheter assembly is advanced through the coronary vasculature until the balloon portion is positioned across the occlusive lesion. Once in position across the lesion, the balloon is inflated to a predetermined size to radially compress against the atherosclerotic plaque of the lesion to remodel the lumen wall. The balloon is then deflated to a smaller profile to allow the catheter to be withdrawn from the patient's vasculature.
A problem associated with the above procedure includes formation of intimal flaps or torn arterial linings which can collapse and occlude the conduit after the balloon is deflated. Moreover, thrombosis and restenosis of the artery may develop over several months after the procedure, which may require another angioplasty procedure or a surgical by-pass operation. To reduce the partial or total occlusion of the artery by the collapse of arterial lining and to reduce the chance of the development of thrombosis and restenosis, a stent is implanted in the lumen to maintain the vascular patency.
Stents are used not only as a mechanical intervention but also as a vehicle for providing biological therapy. As a mechanical intervention, stents act as scaffoldings, functioning to physically hold open and, if desired, to expand the wall of the passageway. Typically, stents are capable of being compressed, so that they can be inserted through small vessels via catheters, and then expanded to a larger diameter once they are at the desired location. Examples in patent literature disclosing stents which have been applied in PTCA procedures include stents described in U.S. Pat. No. 4,733,665 issued to Palmaz, U.S. Pat. No. 4,800,882 issued to Gianturco, and U.S. Pat. No. 4,886,062 issued to Wiktor.
Biological therapy can be achieved by medicating the stents. Medicated stents provide for local administration of a therapeutic substance at the diseased site. In order to provide an efficacious concentration to the treatment site, systemic administration of such medication often leads to adverse or toxic side effects on the patient. Local delivery is a preferred method of treatment in that, in comparison to systemic dosages, smaller total levels of medication can be administered but are concentrated at a specific site. Local delivery thus produces fewer side effects and achieves better results. One proposed method for medicating stents involves the use of a polymeric carrier coated onto the surface of a stent. A blend which includes a solvent, a polymer dissolved in the solvent, and a therapeutic substance dispersed in the blend is applied to the stent. The solvent is allowed to evaporate, leaving on the stent surface a coating of the polymer and the therapeutic substance impregnated in the polymer.
To the extent that the mechanical functionality of stents has been optimized in recent years, continued improvements in the local delivery of drugs by a stent is needed. More specifically, for effective treatment of restenosis, it is important to maintain the concentration of the drug at a therapeutically effective level for a period of time. Hence, controlling a rate of release of the drug from the stent coating is important. In view of the foregoing, coatings for reducing the rate of release of a therapeutic substance from implantable devices, such as stents, are desired.